[50.01] The D/H ratio in Jupiter and Saturn from High-resolution Spectral Observations near 8.6 \mum

On December 2 and 4 2000 UT, we used the mid-infrared
spectrometer Texes mounted at the NASA/Infrared Telescope
Facility (IRTF) to observe Jupiter and Saturn in the 8-13
\mum region. Spectra recorded in the range 1152-1159
cm-1 at an unprecedented resolution of ~ 75\,000
display the Q-branch of CH3D with emission lines of
stratospheric origin in its core. Emission in the cores of
two CH4 lines are also detected in the same spectral
interval. The two sets of lines have similar intensities and
probe a similar pressure range centered around 15 mbar on
Jupiter and 1 mbar on Saturn. These features allow us to
determine the D/H ratio in methane for the two planets with
little sensitivity to uncertainties in the flux calibration
and atmospheric model.

From a preliminary analysis, we derive D/H ratios in methane
of (2.0±0.3)\times10-5 on Jupiter and
(2.4±0.5)\times10-5 on Saturn. Using published
fractionation factors, D/H ratios in hydrogen of
(1.6±0.3)\times10-5 on Jupiter and
(1.8±0.5)\times10-5 on Saturn are inferred. These
results are consistent with those derived from observations
with the Infrared Space Observatory (ISO) at lower spectral
resolution (Lellouch {\em et al.\/} 2001, {\em Astron.\/
Astrophys.\/} 370, 610-622) but bear smaller error
bars. The D/H ratios in Jupiter and Saturn are identical
within error bars, in contrast to previous measurements
which suggested that Jupiter's D/H ratio might exceed that
of Saturn. The Jovian value agrees with that measured in the
Local Interstellar Medium (1.5±0.1 \times10-5),
indicating a very weak consumption of deuterium in the part
of the Galaxy where the Solar System formed over the last
4.5 Gyr.